During hydrocarbon drilling and recovery operations, and in an effort to identify petroleum sources underground, various properties of earth formations are measured by lowering measurement tools into a drilled borehole. For example, field of gravity measurements are taken in boreholes to monitor deep water fluids within underground reservoirs. From a geophysical point of view, a high level of precision and repeatability of borehole measurements determine the success of monitoring earth formations. Data of a high quality obtained through a high quality measurement tool may be contaminated by errors due to a poor level of repeatability.
Depth error of the tool position, i.e., movement of the tool producing variations in depth over repeated measurements, leads to errors in the measured signal. For example, downhole gravity measurements include an additional error term (“δg”) due to an input from the normal gradient (“δg0”) of the gravity field, and also due to variations in the medium in which the measurements are performed (“δgs”). A “Gal” is a unit of gravitational acceleration (“g”) equal to 1 cm/sec2. Typically, this additional term is approximated by a double Bouguer correction. The error δg in tool positioning can be interpreted as a random measurement error depending on rock density and relative displacement.
One approach for reducing the error relating to unknown displacement involves measuring the vertical gradient of the borehole gravity. However, such measurement has various shortcomings, including low signal levels and high locality characteristics. The concept of a locality characteristic of gravity method is strongly linked to the relative contribution of different parts of the investigated medium to the measured signal. The smaller the contribution from the medium parts situated far from the measuring point, the higher the locality feature and vice versa.